Polyvinylidene fluoride (PVDF) is a highly weatherable, chemically resistant engineering plastic. Polyvinylidene film can be difficult to adhere directly to many substrates, such as polyolefins and polyamides without treating the surface of the film with primers or other surface treatments like corona, plasma etc, and/or without the use of adhesives.
Polyvinylidene fluoride has been used in multi-layer fuel hose constructions with polyamides or polyolefins. US 2005-0170,121, and US 2006-0275572 for example. Each of these hose constructions involves thick layers of polymer, bonded with tie layers.
PVDF films have been attached to polyesters, as described in patent applications FR 10.58328, WO 07/085,769, US 2010/0175742 and WO 2011/086318, by using an adhesive. The polyester side of the PVDF/adhesive/polyester constructions is then treated and an adhesive layer added to attach another polyolefin-based film. These PVDF/adhesive/polyester/adhesive/polyolefin constructions are often used as backsheets in a photovoltaic module. Photovoltaic (PV) modules typically consist of a transparent glass or polymer frontsheet, solar cells protected by encapsulation, and a backsheet. The solar cells could be made of crystalline silicon, amorphous silicon, cadmium indium gallium selenide (CIGS), or cadmium indium selenide (CIS), or other similar materials. The backsheet is exposed to the environment on the backside of the module. The primary function of the backsheet is to provide protection to the encapsulated cells from degradation induced by reactions with water, oxygen, and/or UV radiation. The backsheet also provides electrical insulation for the module. Solar cells are commonly encapsulated in ethylene vinyl acetate (EVA), so the backsheet material should adhere well to EVA when the components of the PV are laminated together in a thermoforming process.
The problem with attaching PVDF, and other weatherable films to a polyolefin, as currently practiced in a photovoltaic backsheet is that the costs are high, due to both the expense of the high-performance materials, and the multiple lamination steps in the process. Further, attempting to laminate a very thin (<50, preferably <75 micron) surface treated monolayer PVDF film directly to a polyolefin encapsulant results in wrinkling of the PVDF layer, which can lead to electrical problems in the photovoltaic module.
A thin multi-layer film construction has now been developed, which overcomes the problems described above, having an outer layer of polyvinylidene fluoride or other weatherable polymer, and an inner layer of polyolefin or polyamide. The multi-layer film is capable of adhering to functionalized and non-functionalized polyamide, and/or functionalized and non-functionalized polyolefin substrates, without the need for further surface treatment or coatings/adhesives/tie layers. A substrate having the inventive film adhered thereto, is provided the weathering protection of the polyvinylidene fluoride or other weatherable polymer at a reduced cost. The multi-layer film can be formed in a single unit coextrusion or extrusion coating process.
The purpose of the invention is to provide a multi-layer film with a pre-adhered” polyolefin or polyamide layer on one side and a highly weatherable layer on the other side. This allows for immediate adherence of the film to a polyolefin or polyamide substrate during manufacturing of a structure.
If the polyolefin layer is thick enough, then the multi-layer film structure could be used independently as a backsheet encapsulant in a PV module. A backsheet encapsulant is a single material that serves as both the backsheet and the backside encapsulant layer in a photovoltaic module.